The electrically erasable programmable read-only memory (hereinafter referred to as EEPROM) has proven useful because it combines the convenience of non-volatile data storage with the capability to erase the stored data electrically, then program new data. The utility of EEPROM has been increased by the development of various types of flash EEPROM, which erase a large number of memory cells simultaneously. There is a strong interest in flash EEPROM with megabit storage capacity for use in devices such as portable computers.
The basic EEPROM memory cell has a field-effect transistor with a control gate and a floating gate. The cell is erased and programmed by producing a potential difference between the control gate and the source or drain large enough to cause Fowler-Nordheim tunneling of electrons into or out of the floating gate. A high voltage required for the tunneling is generated within the EEPROM chip.
For example, one prior-art EEPROM has a charge pump by which it generates 20 V internally, using the potential difference between this voltage and ground (0 V) for erasing and programming. Another prior-art EEPROM has a negative charge pump that generates −15 V internally, using the 20 V potential difference between this and the 5 V supply voltage for erasing and programming. Still another prior-art EEPROM produces 18 V and −11 V internally, using the potential difference between −11 V and the supply voltage (5 V) for erasing and the potential difference between 18 V and ground (0 V) for programming.
A problem with these prior-art EEPROMs is that the transistors that deliver voltages such as 20 V, 18 V, −11 V and −15 V to the memory cell must be large in size. To provide high breakdown voltages, these transistors require deep junctions and thick gate oxides, or increased gate lengths. Occupying excessive space on the EEPROM chip, they make it difficult to attain the high levels of integration desired for many applications.
A related problem is that longer-than-normal wafer processes are needed to fabricate these high-voltage transistors. Thus they add to the time and cost of the EEPROM manufacturing process.
A further problem, particularly in flash EEPROMs that erase all memory cells simultaneously, is that delivery of a high erasing voltage to a memory cell that is already erased can drive the floating gate of the memory cell into a depletion mode, causing the cell to leak and produce false data. Complex schemes have been necessary to avoid such overerasing.